FIG. 7 is one example of semiconductor integrated circuits for transmission incorporated into a prior art handset for wireless mobile communications (hereinafter, referred to as a handset).
The semiconductor integrated circuits (hereinafter, IC) are produced discretely for each function. Each of the parts enclosed by a square is a discrete IC. The numeral 70 denotes a protection circuit against electrostatic discharge (hereinafter, referred to as a protection circuit) of the discrete ICs.
In the prior art construction, the protection circuits 70 are provided in an automatic gain-control amplifiers 73 and 76, a demodulator 74, and a modulator 75, which are intermediate frequency band ICs of several hundred MHz. The protection circuit 70 prevents circuit damage due to electrostatic charge from several tens of V to several hundreds of V. On the other hand, the protection circuits are not provided in a low-noise amplifier 71, a receiving mixer 72, a transmitting mixer 77, a pre-amplifier for transmission 78, and a high-power amplifier 79, which are high frequency band ICs in the vicinity of 1 GHz.
Japanese Published Unexamined Patent Application No. Hei 07-202583 discloses one example of a protection circuit. This is as shown in FIG. 8. The numeral 81 denotes a protection circuit. V denotes an input or output signal to/from an integrated circuit, which is inputted through a signal line 84 to an internal circuit 85 or is outputted therethrough from the internal circuit 85. The protection circuit 81 has diode-connected NMOS transistors 82 and 83. The gate and drain of the transistor 82 are short-circuited and are connected to the signal line 84. The source thereof is connected to power source Vcc. The drain of the transistor 83 is connected to the signal line 84, and the gate and source thereof are short-circuited and are connected to a ground. The top and bottom diagrams on the right side of FIG. 8 respectively show the static characteristics of the transistors 82 and 83 and the operation of a high-frequency signal. When positive static electricity V1 higher than voltage Von+power source voltage Vcc is applied to the signal line 84, forward current of diode I is then flowed to the transistor 82 so as to protect the internal circuit 85. When high static voltage V2 is applied, a breakdown current is flowed to the transistor 83.
Upon application of static voltage V3′ below −Von to the signal line 84, forward current of diode I′ is flowed to the transistor 83. Further, upon application of negative high static voltage V4′ is applied, a breakdown current is flowed to the transistor 82.
Japanese Published Unexamined Patent Application No. Hei 01-230266 discloses one example of a protection circuit constructed by diodes connected by multiple stages. FIGS. 9 and 10 respectively illustrate a circuit diagram thereof and a cross section of a diode construction. Diodes 91 and 92 are protection circuits. A signal is transmitted to a circuit 94 by a signal line 93. A protection circuit between the power source line and the signal line 93 is omitted. Since the diodes are connected by two stages, the Von voltage is high. FIG. 10 shows the cross-sectional construction of the protection circuit in which the diodes are connected by two stages. The numeral 101 denotes a P type substrate forming an integrated circuit. The numeral 102 denotes an N type epitaxial layer. A P type diffusion layer 103 and an N type diffusion layer 104 form a PN junction diode. A P type isolation diffusion layer 105 is a channel stopper.
The numeral 106 denotes a silicon dioxide film 106. A cathode electrode 107 is grounded to a signal line 109 of the internal circuit, and an anode electrode 108 is grounded outside the integrated circuit.